(Not Applicable)
The invention generally relates to compositions, articles, and methods for packaging oxygen-sensitive substances, especially comestible products. The invention is directed to oxygen barrier materials having improved passive oxygen barrier properties and also having active oxygen scavenger properties. The active oxygen scavengers of this invention are condensation copolymeric substances which can be used for bottles and packaging and have the ability to consume, deplete or reduce the amount of oxygen in or from a given environment in the solid state at ambient temperatures. Formulations are disclosed which may be fabricated into plastic bottles and other packaging articles and films.
Plastic materials have continued to make significant advancements into the packaging industry due to the design flexibility of their material and their ability to be fabricated in various sizes and shapes commonly used in the packaging industry. The deployment of plastic materials into packaging articles such as films, trays, bottles, cups, bowls, coatings and liners is already commonplace in the packaging industry. Although plastic materials offer the packaging industry many benefits with an unlimited degree of design flexibility, the utility of plastic materials has remained inhibited in situations where barrier properties to atmospheric gases (primarily oxygen) are necessary to assure adequate product shelf life. When compared to traditional packaging materials such as glass and steel, plastics offer inferior barrier properties which limits their acceptability for use in packaging items that are sensitive to atmospheric gases, particularly when the exposure to the atmospheric gases will entail extended time periods. The packaging industry continues to seek packaging materials which offer the design flexibility of plastics and at the same time have the barrier properties of glass and steel.
The packaging industry has developed technology to improve the barrier properties of plastic containers by developing containers that offer improved barrier properties approaching, but not comparable to, those of glass, steel, and aluminum. By a very wide margin, polyethylene terephthalate (PET) and similar packaging polyesters have gained wide acceptance, especially for bottling applications, in view of the clarity and rigidity associated with PET bottles. PET has made significant inroads into bottling and packaging applications at the expense of the use of glass containers but primarily in applications where the needs for barrier properties are modest. A significant example is the use of PET for soft drink bottles. However, PET barrier properties have limited its use in the packaging of oxygen sensitive products.
It is generally accepted in the packaging industry that polyamides have superior passive oxygen barrier properties when compared to similar polyester packaging constructions. A useful passive oxygen barrier polymer is one that exhibits the ability to retard the permeability of oxygen through it when compared with the permeability of oxygen through other resins. Further, it has been reported that a polyamide known as MXD-6 has some active oxygen barrier capacity. MXD6 is poly(m-xyleneadipamide) which is a polyamide made from equal molar amounts of the two monomers (1) meta-xylenediamine and (2) adipic acid. An active oxygen barrier resin is a substance capable of intercepting and scavenging oxygen (by undergoing chemical reaction with the oxygen) as it attempts to pass through the packaging. This method also affords the opportunity to eliminate unwanted oxygen from within the package cavity wherein said oxygen may have been inadvertently introduced during packaging or filling. This method of providing oxygen barrier properties where a substance consumes or reacts with the oxygen is known as an xe2x80x9cactive oxygen barrierxe2x80x9d and is a different concept from passive oxygen barriers which attempt to hermetically seal a product away from oxygen via the passive approach.
When MXD-6 (about 4 wt %) is blended with PET (about 96 wt %), the resulting blend is about 70% as permeable to oxygen as a similar construction of unmodified PET. Presumably, this 30% improvement over unmodified PET may be attributed to the improvement in passive barrier properties of the aforementioned blend. When an oxidation catalyst is added to the blend (e.g., about 50-200 PPM cobalt with respect to the weight of the blend), the blend takes on enhanced active oxygen scavenging properties. The O2 permeability of the blend is diminished under these conditions until the active O2 scavenging capacity of the blend is depleted. The barrier properties achieved by the blend are suitable only for less demanding packaging requirements and then only with very heavy use of the blend. However, MXD-6 is a relatively expensive polyamide and the use of large amounts of it in a package serves to undermine the economic viability of such packaging. Lower cost, more common polyamides, such as the well known poly(hexamethyleneadipamide) have the improved passive barrier properties of polyamides but are devoid of active barrier properties. What is needed is an active-passive polyamide oxygen barrier polyamide-based resin which may be produced at reasonable cost and which has sufficient oxygen scavenging and barrier properties to offer the possibility of target shelf lives in the range of 6 months to two years for oxygen sensitive products. This invention addresses such need.
In a commonly assigned and co-pending application filed on Sep. 23, 1996 and having Ser. No. 08/717,370, it was disclosed that certain hydrocarbons, such as polyolefins, (especially polydienes) when present in small amounts as polyolefin oligomer blocks in a block copolyester polymer added substantial active oxygen scavenging capacity to packaging polyesters which showed no active oxygen scavenging capacity what-so-ever in the absence of the polyolefin oligomer blocks. The oxygen scavenging copolyesters of the above-referenced application were comprised predominantly of packaging polyester segments with only an oxygen scavenging amount of polyolefin oligomer segments present to supply the oxygen scavenging capacity required for the intended packaging application. The copolyesters of the application having Ser. No. 08/717,370 were typically in the range of about 0.5-12 wt % polyolefin oligomer segments with the remainder comprising polyester segments. An especially preferred embodiment was a copolyester of about 4 wt % polyolefin oligomer segments with the remainder being polyester segments. Such block copolyesters comprising low weight percent levels of polyolefin oligomer segments have properties (such as melting point, viscosity, and clarity) very similar to the unmodified polyester from which the polyester segments were derived. In particular, layers in laminar packages and bottles having one or several layers of unmodified polyester and one or several layers of oxygen scavenging block copolyester as described above, were self-adherent and packaging articles appeared to be a monolithic (rather than layered) construction.
For this invention, applicants have extended the concept of implanting high capacity oxygen scavenging polyolefin oligomer segments into polyamides forming block copolyamides comprising predominantly polyamide segments and an oxygen scavenging amount of polyolefin oligomer segments. As was the case for the copolyesters disclosed in the application having Ser. No. 08/717,370, the copolyamides of this invention have properties very similar to the polyamide from which the polyamide segments were derived. A typical use for such polyamides comprises a layered construction such as a package film or bottle wall having outer and inner layers of polyamide and a middle layer of copolyamide (wherein the polyamide segments of the copolyamide are derived from those of the inner and/or outer layer polyamides and the oxygen scavenging segments comprise a polyolefin oligomer). This arrangement serves to provide properties for the copolyamide layer which are very similar to the properties of the unmodified polyamide layers which is an important concept of this invention for laminar constructions. A major concept of this invention, however, is the incorporation of highly efficient oxygen scavenging polyolefin oligomer segments into the copolyamide while leaving the copolyamide with properties very similar to the unmodified polyamide. The high active oxygen scavenging capacity of the copolyamides disclosed is derived from the active oxygen scavenging capacity of the polyolefin oligomer segments. As noted previously, polyamides, per se, are generally considered to have superior passive oxygen barriers properties as compared to polyesters. Thus, another important concept of this invention is the combination of superior passive barrier properties with active oxygen scavenging capacity when compared to the use of unmodified polyester alone or unmodified polyamide alone.
An active oxygen barrier resin is a substance capable of intercepting and scavenging oxygen (by undergoing chemical reaction with the oxygen) as it attempts to pass through the packaging. Active oxygen scavenging also affords the opportunity to eliminate unwanted oxygen (often called head space oxygen) from within the package cavity wherein said oxygen may have been inadvertently introduced during packaging or filling. This method of providing oxygen barrier properties where a substance consumes or reacts with the oxygen is known as an xe2x80x9cactive oxygen barrierxe2x80x9d and is a different concept from passive oxygen barriers which attempt to physically seal a product away from oxygen via the passive approach. Only active oxygen scavengers can remove unwanted oxygen (inadvertently introduced during packaging) from the package cavity. Active oxygen scavenging implies, therefore, consumption of a material incorporated in the wall of a package. The material is progressively consumed so that the active oxygen scavenging ability is eventually depleted or at least diminished. However, this eventual depletion of the active oxygen scavenging moiety can be adjusted so that the depletion occurs only well after the required oxygen free shelf life of the packaged product which is typically one year or less.
U.S. Pat. No. 5,021,515 (CMB Patent) discloses CMB""s OxBar oxygen scavenging system. The CMB Patent is directed to the use of a polyamide (blended with polyester) as an active oxygen scavenger moiety. The CMB Patent discloses the use of a polyamide blended with a bottling polyester such as PET and further requires the presence of a catalyst, such as a transition metal. Such blends are subsequently deployed so as to comprise at least one layer in a single or multi-layer package or bottle wall. According to the CMB Patent, the polyamide in the blend is the moiety responsible for the active oxygen scavenging capacity. of the blend. In a preferred embodiment of the CMB Patent, 96 wt % PET is blended with 4 wt % of a polyamide frequently designated as MXD6. MXD6 is a polyamide made from equal molar amounts of the two monomers (1) metaxylene diamine and (2) adipic acid. The PET/MXD6 blend is typically deployed in the presence of about 200 PPM of cobalt which serves to catalyze the active oxygen scavenging function.
EP-A-0 507 207 discloses a composition for scavenging oxygen comprising an ethylenically unsaturated hydrocarbon polymer and a transition metal catalyst.
The current invention is directed to the use of copolyamides capable of scavenging oxygen in the solid state comprising predominately polyamide segments and an oxygen scavenging amount of polyolefin oligomer segments. The copolyamides of this invention are typically deployed in the presence of a catalyst, such as a transition metal, and comprise at least one layer of a single or multi-layer wall of a package or bottle. Significant differences between this invention and the CMB Patent include (1) the current invention is directed to a copolyamide comprising predominantly polyamide segments while the CMB Patent discloses a polyester/polyamide blend which is predominantly polyester (the CMB Patent does not disclose the use of polyolefin what-so-ever), (2) the polyolefin oligomer segments in the copolyamides of this invention are the moieties which react with and scavenge the oxygen whereas in the CMB Patent the polyamide reacts with and scavenges the oxygen, (3) the oxygen scavenging ability of the copolyamides of this invention are substantially greater than those of the PET/MXD6 blend, and (4) the copolyamides of this invention are typically used in polyamide based packages and bottles where as the PET/MXD-6 blend is aimed at polyester (PET) based packages and bottles.